In the production of automotive lamp lenses, particularly head lamp lenses, acrylic resins such as polymethyl methacrylate (PMMA), and acrylic-styrene resins (AS) have conventionally been used. In recent years, however, polycarbonate resins having high mechanical strength and heat resistance have come to cause attention and are practically used for the above use.
However, polycarbonate resins alone have problems concerning weatherability and mar resistance. Although a secondary processing to form a coating having good weatherability and mar resistance is mainly conducted to overcome those problems, this coating treatment necessitates a residual stress-reducing step, in which molded articles to be coated are heated with hot-air flow to remove molding strain in order to prevent blushing and cracking during the formation of the coating and also to prevent cracking due to long-term fatigue, and a cleaning and drying step for imparting good adhesion and appearance to the coating. In the residual stress-reducing step of those steps, a hot-air circulating drying oven is used for hot-air flow, but use of the hot-air circulating drying oven has been disadvantageous in that residual stress reduction takes much time, resulting in increased production cost, limited treating capacity, etc.